This invention relates to two-wire papermaking apparatus. More particularly, it relates to two-wire paper forming apparatus especially designed to form so-called lightweight coated grades of paper (i.e. lightweight coating rawstock or coating rawstock) at high speeds such as, for example, 3,500 feet per minute or greater. Still more particularly, this invention relates to two-wire paper forming apparatus wherein the forming wires define a curved forming zone.
Presently, lightweight coated grades of paper (LWC) are made on papermaking machine which utilize a fourdrinier forming zone. While the fourdrinier arrangement can produce this grade paper at relatively high speeds--such as, for example, about 3,000 feet per minute, the single fourdrinier forming wire inherently produces what is known in the trade as a two-sided sheet. In other words, the sheet has different surface properties and appearance, such as wire marks and different proportions of fines and fillers on either side of the sheet. LWC grades, which generally range from about 30-48 grams/m.sup.2, are commonly used for magazine paper, and the demand for such paper has increased markedly in the past few years as has the requirement that both sides of the sheet have equal coating and printability properties. This means that the paper web be as close to one-sided as possible. This one-sided characteristic simply cannot be produced on fourdrinier machines because the water drainage during the formation process is in one direction only so the resulting distribution of fines, fibers and fillers is not equal on both sides of the web. Further, the wire marks only the bottom side of the web which exacerbates the differences, or two-sided nature of paper webs formed on fourdrinier type machines.
In addition, other two-wire paper forming machines also are deficient in their suitability for forming LWC paper grades due to their arrangement of the various elements which contribute to the manner in which water is urged from the stock, the duration of this water removal process, the sequence of the application of the water removal elements to either side of the forming zone and the shape of the forming zone itself.
Thus, in prior two-wire forming arrangements, an excessive proportion of the fines and fillers, which greatly contribute to the overall quality of a coated paper sheet, were removed initially in the paper forming process. Once removed, they could not be subsequently replaced in the web as it is formed, nor could their absence be entirely compensated for by other means, such as applying excess coating.
However, if it is attempted to retain the fines and fillers in the paper web by not subjecting it to strong water removal forces during its initial stages of formation, the web cannot be produced at an economically competitive speed.